Which serial interface is right for your application?
There are dozens of serial interfaces available, but RS232 and RS485 are some of the most common go to for many of our mass flow applications. Here, we go over the important differences between the two interfaces: communication distance, number of devices being used, level of electrical noise, and network topology.
The maximum cable distance for RS232 should be 50ft according to the EIA/TIA-232-F specification. It can be difficult to maintain signal entirety beyond this. RS485 on the other hand has an upper limit of 4,000ft.
Number of devices
RS232 is intended to be used as point to point communication. This means a single master and a single slave device. Now because we are Alicat, we can be flexible in order to help our customers. Rarely, we have set up a multidrop communication, but this is uncommon and many other manufacturers do not use this set up, so don’t mix and match.
RS485 was designed to work with up to 32 other devices.
Electrical noise and ground reference variations
RS232 is voltage based and uses ground as its reference point. This means that if you see some electrical noise on your ground line this can be interpreted as a part of the signal.
RS485 on the other hand has higher noise immunity. It is based off of a differential voltage system and is more suitable to high noise environments. This is done is by using two wires both referenced to each other. If ground shifts both signal lines shift in the same direction keeping the same relative distance from one another.
Network topology and data speed
Care must be taken setting up a RS485 network. Unlike RS232, RS485 requires the use of a daisy chain topology and will likely cause issues if setup in a star. Termination resistors are also required at the first device in the chain and on the last device in the chain to help with reflections. The data speed allowed is dependent on the length of cables / topology.
In general, RS232 is beginning to fall out of favor because it is being replaced by USB for short distances even though it takes a bit more number crunching power of the processor. While most modern PCs have phased out the RS232 serial port in favor of USB setting up a network is much simpler. USB to RS232 converters are also readily available online.